Compartmentalization of neuronal and peripheral serotonin synthesis in Drosophila melanogaster

Distribution analysis of TRH in Bactrocera dorsalis using a CRISPR/Cas9-mediated reporter knock-in strain

Although the study of many genes and their protein products is limited by the availability of high-quality antibodies, this problem could be solved by fusing a tag/reporter to an endogenous gene using a gene-editing approach. The type II bacterial CRISPR/Cas system has been demonstrated to be an efficient gene-targeting technology for many insects, including the oriental fruit fly Bactrocera dorsalis. However, knocking in, an important editing method of the CRISPR/Cas9 system, has lagged in its application in insects. Here, we describe a highly efficient homology-directed genome editing system for B. dorsalis that incorporates coinjection of embryos with Cas9 protein, guide RNA and a short single-stranded oligodeoxynucleotide donor. This one-step procedure generates flies carrying V5 tag (42 bp) in the BdorTRH gene. In insects, as in other invertebrates and in vertebrates, the neuronal tryptophan hydroxylase (TRH) gene encodes the rate-limiting enzyme for serotonin biosynthesis in the central nervous system. Using V5 monoclonal antibody, the distribution of TRH in B. dorsalis at different developmental stages was uncovered. Our results will facilitate the generation of insects carrying precise DNA inserts in endogenous genes and will lay foundation for the investigation of the neural mechanisms underlying the serotonin-mediated behaviour of B. dorsalis.

Decoding bee cleptoparasitism through comparative transcriptomics of Coelioxoides waltheriae and its host Tetrapedia diversipes

Cleptoparasitism, also known as brood parasitism, is a widespread strategy among bee species in which the parasite lays eggs into the nests of the host species. Even though this behavior has significant ecological implications for the dynamics of several species, little is known about the molecular pathways associated with cleptoparasitism. To shed some light on this issue, we used gene expression data to perform a comparative analysis between two solitary neotropical bees: Coelioxoides waltheriae, an obligate parasite, and their specific host Tetrapedia diversipes. We found that ortholog genes involved in signal transduction, sensory perception, learning, and memory formation were differentially expressed between the cleptoparasite and the host. We hypothesize that these genes and their associated molecular pathways are engaged in cleptoparasitism-related processes and, hence, are appealing subjects for further investigation into functional and evolutionary aspects of cleptoparasitism in bees.

Lessons from lonely flies: Molecular and neuronal mechanisms underlying social isolation

Animals respond to changes in the environment which affect their internal state by adapting their behaviors. Social isolation is a form of passive environmental stressor that alters behaviors across animal kingdom, including humans, rodents, and fruit flies. Social isolation is known to increase violence, disrupt sleep and increase depression leading to poor mental and physical health. Recent evidences from several model organisms suggest that social isolation leads to remodeling of the transcriptional and epigenetic landscape which alters behavioral outcomes. In this review, we explore how manipulating social experience of fruit fly Drosophila melanogaster can shed light on molecular and neuronal mechanisms underlying isolation driven behaviors. We discuss the recent advances made using the powerful genetic toolkit and behavioral assays in Drosophila to uncover role of neuromodulators, sensory modalities, pheromones, neuronal circuits and molecular mechanisms in mediating social isolation. The insights gained from these studies could be crucial for developing effective therapeutic interventions in future.

Characterization of a novel RNAi yeast insecticide that silences mosquito 5-HT1 receptor genes

G protein-coupled receptors (GPCRs), which regulate numerous intracellular signaling cascades that mediate many essential physiological processes, are attractive yet underexploited insecticide targets. RNA interference (RNAi) technology could facilitate the custom design of environmentally safe pesticides that target GPCRs in select target pests yet are not toxic to non-target species. This study investigates the hypothesis that an RNAi yeast insecticide designed to silence mosquito serotonin receptor 1 (5-HTR1) genes can kill mosquitoes without harming non-target arthropods. 5-HTR.426, a Saccharomyces cerevisiae strain that expresses an shRNA targeting a site specifically conserved in mosquito 5-HTR1 genes, was generated. The yeast can be heat-inactivated and delivered to mosquito larvae as ready-to-use tablets or to adult mosquitoes using attractive targeted sugar baits (ATSBs). The results of laboratory and outdoor semi-field trials demonstrated that consumption of 5-HTR.426 yeast results in highly significant mortality rates in Aedes, Anopheles, and Culex mosquito larvae and adults. Yeast consumption resulted in significant 5-HTR1 silencing and severe neural defects in the mosquito brain but was not found to be toxic to non-target arthropods. These results indicate that RNAi insecticide technology can facilitate selective targeting of GPCRs in intended pests without impacting GPCR activity in non-targeted organisms. In future studies, scaled production of yeast expressing the 5-HTR.426 RNAi insecticide could facilitate field trials to further evaluate this promising new mosquito control intervention.

Multiomic interpretation of fungus-infected ant metabolomes during manipulated summit disease

Camponotus floridanus ants show altered behaviors followed by a fatal summiting phenotype when infected with manipulating Ophiocordyceps camponoti-floridani fungi. Host summiting as a strategy to increase transmission is also observed with parasite taxa beyond fungi, including aquatic and terrestrial helminths and baculoviruses. The drastic phenotypic changes can sometimes reflect significant molecular changes in gene expression and metabolite concentrations measured in manipulated hosts. Nevertheless, the underlying mechanisms still need to be fully characterized. To investigate the small molecules producing summiting behavior, we infected C. floridanus ants with O. camponoti-floridani and sampled their heads for LC-MS/MS when we observed the characteristic summiting phenotype. We link this metabolomic data with our previous genomic and transcriptomic data to propose mechanisms that underlie manipulated summiting behavior in "zombie ants." This "multiomic" evidence points toward the dysregulation of neurotransmitter levels and neuronal signaling. We propose that these processes are altered during infection and manipulation based on (1) differential expression of neurotransmitter synthesis and receptor genes, (2) altered abundance of metabolites and neurotransmitters (or their precursors) with known behavioral effects in ants and other insects, and (3) possible suppression of a connected immunity pathway. We additionally report signals for metabolic activity during manipulation related to primary metabolism, detoxification, and anti-stress protectants. Taken together, these findings suggest that host manipulation is likely a multi-faceted phenomenon, with key processes changing at multiple levels of molecular organization.

Navigating Like a Fly: Drosophila melanogaster as a Model to Explore the Contribution of Serotonergic Neurotransmission to Spatial Navigation

Serotonin is a monoamine that acts in vertebrates and invertebrates as a modulator promoting changes in the structure and activity of brain areas relevant to animal behavior, ranging from sensory perception to learning and memory. Whether serotonin contributes in Drosophila to human-like cognitive abilities, including spatial navigation, is an issue little studied. Like in vertebrates, the serotonergic system in Drosophila is heterogeneous, meaning that distinct serotonergic neurons/circuits innervate specific fly brain regions to modulate precise behaviors. Here we review the literature that supports that serotonergic pathways modify different aspects underlying the formation of navigational memories in Drosophila.

Serotonergic control of feeding microstructure in Drosophila

To survive, animals maintain energy homeostasis by seeking out food. Compared to freely feeding animals, food-deprived animals may choose different strategies to balance both energy and nutrition demands, per the metabolic state of the animal. Serotonin mediates internal states, modifies existing neural circuits, and regulates animal feeding behavior, including in humans and fruit flies. However, an in-depth study on the neuromodulatory effects of serotonin on feeding microstructure has been held back for several technical reasons. Firstly, most feeding assays lack the precision of manipulating neuronal activity only when animals start feeding, which does not separate neuronal effects on feeding from foraging and locomotion. Secondly, despite the availability of optogenetic tools, feeding in adult fruit flies has primarily been studied using thermogenetic systems, which are confounded with heat. Thirdly, most feeding assays have used food intake as a measurement, which has a low temporal resolution to dissect feeding at the microstructure level. To circumvent these problems, we utilized OptoPAD assay, which provides the precision of optogenetics to control neural activity contingent on the ongoing feeding behavior. We show that manipulating the serotonin circuit optogenetically affects multiple feeding parameters state-dependently. Food-deprived flies with optogenetically activated and suppressed serotonin systems feed with shorter and longer sip durations and longer and shorter inter-sip intervals, respectively. We further show that serotonin suppresses and enhances feeding via 5-HT1B and 5-HT7 receptors, respectively.

Regulation and modulation of biogenic amine neurotransmission in Drosophila and Caenorhabditis elegans

Neurotransmitters are crucial for the relay of signals between neurons and their target. Monoamine neurotransmitters dopamine (DA), serotonin (5-HT), and histamine are found in both invertebrates and mammals and are known to control key physiological aspects in health and disease. Others, such as octopamine (OA) and tyramine (TA), are abundant in invertebrates. TA is expressed in both Caenorhabditis elegans and Drosophila melanogaster and plays important roles in the regulation of essential life functions in each organism. OA and TA are thought to act as the mammalian homologs of epinephrine and norepinephrine respectively, and when triggered, they act in response to the various stressors in the fight-or-flight response. 5-HT regulates a wide range of behaviors in C. elegans including egg-laying, male mating, locomotion, and pharyngeal pumping. 5-HT acts predominantly through its receptors, of which various classes have been described in both flies and worms. The adult brain of Drosophila is composed of approximately 80 serotonergic neurons, which are involved in modulation of circadian rhythm, feeding, aggression, and long-term memory formation. DA is a major monoamine neurotransmitter that mediates a variety of critical organismal functions and is essential for synaptic transmission in invertebrates as it is in mammals, in which it is also a precursor for the synthesis of adrenaline and noradrenaline. In C. elegans and Drosophila as in mammals, DA receptors play critical roles and are generally grouped into two classes, D1-like and D2-like based on their predicted coupling to downstream G proteins. Drosophila uses histamine as a neurotransmitter in photoreceptors as well as a small number of neurons in the CNS. C. elegans does not use histamine as a neurotransmitter. Here, we review the comprehensive set of known amine neurotransmitters found in invertebrates, and discuss their biological and modulatory functions using the vast literature on both Drosophila and C. elegans. We also suggest the potential interactions between aminergic neurotransmitters systems in the modulation of neurophysiological activity and behavior.

Mutation of the Drosophila melanogaster serotonin transporter dSERT impacts sleep, courtship, and feeding behaviors

The Serotonin Transporter (SERT) regulates extracellular serotonin levels and is the target of most current drugs used to treat depression. The mechanisms by which inhibition of SERT activity influences behavior are poorly understood. To address this question in the model organism Drosophila melanogaster, we developed new loss of function mutations in Drosophila SERT (dSERT). Previous studies in both flies and mammals have implicated serotonin as an important neuromodulator of sleep, and our newly generated dSERT mutants show an increase in total sleep and altered sleep architecture that is mimicked by feeding the SSRI citalopram. Differences in daytime versus nighttime sleep architecture as well as genetic rescue experiments unexpectedly suggest that distinct serotonergic circuits may modulate daytime versus nighttime sleep. dSERT mutants also show defects in copulation and food intake, akin to the clinical side effects of SSRIs and consistent with the pleomorphic influence of serotonin on the behavior of D. melanogaster. Starvation did not overcome the sleep drive in the mutants and in male dSERT mutants, the drive to mate also failed to overcome sleep drive. dSERT may be used to further explore the mechanisms by which serotonin regulates sleep and its interplay with other complex behaviors.

Vibrio cholerae high cell density quorum sensing activates the host intestinal innate immune response

Quorum sensing fundamentally alters the interaction of Vibrio cholerae with aquatic environments, environmental hosts, and the human intestine. At high cell density, the quorum-sensing regulator HapR represses not only expression of cholera toxin and the toxin co-regulated pilus, virulence factors essential in human infection, but also synthesis of the Vibrio polysaccharide (VPS) exopolysaccharide-based matrix required for abiotic and biotic surface attachment. Here, we describe a feature of V. cholerae quorum sensing that shifts the host-pathogen interaction toward commensalism. By repressing pathogen consumptive anabolic metabolism and, in particular, tryptophan uptake, V. cholerae HapR stimulates host intestinal serotonin production. This, in turn, activates host intestinal innate immune signaling to promote host survival.

Transcriptome and Metabolome Analysis Reveals the Importance of Amino-Acid Metabolism in Spodoptera Frugiperda Exposed to Spinetoram

Pests are inevitably exposed to sublethal and lethal doses in the agroecosystem following the application of pesticides indispensable to protect food sources. The effect of spinetoram on amino-acid metabolism of fall armyworm, Spodoptera frugiperda (J.E. Smith), was investigated, at the dose of LC10 and LC90, by transcriptome and LC-MS/MS analysis. Using statistics-based analysis of both POS and NEG mode, a total of 715,501 metabolites in S. frugiperda were significantly changed after spinetoram treatment. The enhancement of glucose metabolism provides energy support for detoxification in larvae. The decrease in valine and isoleucine is associated with an increase in leucine, without maintaining the conservation of citric acid in the larvae. The down-regulation of phenylalanine may retard the tricarboxylic acid cycle to produce GTP. The abundance of lysine was decreased in response to spinetoram exposure, which damages the nervous system of the larvae. The abundance of arginine increases and causes non-functional contraction of the insect's muscles, causing the larva to expend excess energy. Tryptophan provides an important substrate for eliminating ROS. The changes in glutamic acid, aspartic acid, and lysine cause damage to the nerve centers of the larvae. The results of transcriptome and LC-MS/MS analysis revealed the effects of pesticide exposure on amino-acid metabolism of S. frugiperda successfully and provide a new overview of the response of insect physio-biochemistry against pesticides.

Single cell RNA-seq analysis reveals temporally-regulated and quiescence-regulated gene expression in Drosophila larval neuroblasts

The mechanisms that generate neural diversity during development remains largely unknown. Here, we use scRNA-seq methodology to discover new features of the Drosophila larval CNS across several key developmental timepoints. We identify multiple progenitor subtypes - both stem cell-like neuroblasts and intermediate progenitors - that change gene expression across larval development, and report on new candidate markers for each class of progenitors. We identify a pool of quiescent neuroblasts in newly hatched larvae and show that they are transcriptionally primed to respond to the insulin signaling pathway to exit from quiescence, including relevant pathway components in the adjacent glial signaling cell type. We identify candidate "temporal transcription factors" (TTFs) that are expressed at different times in progenitor lineages. Our work identifies many cell type specific genes that are candidates for functional roles, and generates new insight into the differentiation trajectory of larval neurons.

Sensing of the non-essential amino acid tyrosine governs the response to protein restriction in Drosophila

The intake of dietary protein regulates growth, metabolism, fecundity and lifespan across various species, which makes amino acid (AA)-sensing vital for adaptation to the nutritional environment. The general control nonderepressible 2 (GCN2)-activating transcription factor 4 (ATF4) pathway and the mechanistic target of rapamycin complex 1 (mTORC1) pathway are involved in AA-sensing. However, it is not fully understood which AAs regulate these two pathways in living animals and how they coordinate responses to protein restriction. Here we show in Drosophila that the non-essential AA tyrosine (Tyr) is a nutritional cue in the fat body necessary and sufficient for promoting adaptive responses to a low-protein diet, which entails reduction of protein synthesis and mTORC1 activity and increased food intake. This adaptation is regulated by dietary Tyr through GCN2-independent induction of ATF4 target genes in the fat body. This study identifies the Tyr-ATF4 axis as a regulator of the physiological response to a low-protein diet and sheds light on the essential function of a non-essential nutrient.

Serotonin modulates insect gut bacterial community homeostasis

Background

Metazoan guts are in permanent contact with microbial communities. However, the host mechanisms that have developed to manage the dynamic changes of these microorganisms and maintain homeostasis remain largely unknown.

Results

Serotonin (5-hydroxytryptamine [5-HT]) was found to modulate gut microbiome homeostasis via regulation of a dual oxidase (Duox) gene expression in both Bactrocera dorsalis and Aedes aegypti. The knockdown of the peripheral 5-HT biosynthetic gene phenylalanine hydroxylase (TPH) increased the expression of Duox and the activity of reactive oxygen species, leading to a decrease in the gut microbiome load. Moreover, the TPH knockdown reduced the relative abundance of the bacterial genera Serratia and Providencia, including the opportunistic pathogens, S. marcescens and P. alcalifaciens in B. dorsalis. Treatment with 5-hydroxytryptophan, a precursor of 5-HT synthesis, fully rescued the TPH knockdown-induced phenotype.

Conclusions

The findings reveal the important contribution of 5-HT in regulating gut homeostasis, providing new insights into gut–microbe interactions in metazoans.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01319-x.

Serotonin Signals Modulate Mushroom Body Output Neurons for Sustaining Water-Reward Long-Term Memory in Drosophila

Memory consolidation is a time-dependent process through which an unstable learned experience is transformed into a stable long-term memory; however, the circuit and molecular mechanisms underlying this process are poorly understood. The Drosophila mushroom body (MB) is a huge brain neuropil that plays a crucial role in olfactory memory. The MB neurons can be generally classified into three subsets: γ, αβ, and α′β′. Here, we report that water-reward long-term memory (wLTM) consolidation requires activity from α′β′-related mushroom body output neurons (MBONs) in a specific time window. wLTM consolidation requires neurotransmission in MBON-γ3β′1 during the 0–2 h period after training, and neurotransmission in MBON-α′2 is required during the 2–4 h period after training. Moreover, neurotransmission in MBON-α′1α′3 is required during the 0–4 h period after training. Intriguingly, blocking neurotransmission during consolidation or inhibiting serotonin biosynthesis in serotoninergic dorsal paired medial (DPM) neurons also disrupted the wLTM, suggesting that wLTM consolidation requires serotonin signals from DPM neurons. The GFP Reconstitution Across Synaptic Partners (GRASP) data showed the connectivity between DPM neurons and MBON-γ3β′1, MBON-α′2, and MBON-α′1α′3, and RNAi-mediated silencing of serotonin receptors in MBON-γ3β′1, MBON-α′2, or MBON-α′1α′3 disrupted wLTM. Taken together, our results suggest that serotonin released from DPM neurons modulates neuronal activity in MBON-γ3β′1, MBON-α′2, and MBON-α′1α′3 at specific time windows, which is critical for the consolidation of wLTM in Drosophila.

GC/TOF-MS-Based Metabolomics Reveals Altered Metabolic Profiles in Wood-Feeding Termite Coptotermes formosanus Shiraki Digesting the Weed Mikania micrantha Kunth

Effective approaches to exploiting the biomass of the abundant invasive weed Mikania micrantha Kunth are limited. Termites have been a focus of significant attention as mediators of biomass-processing owing to their ability to digest lignocellulose. Here, the GC/TOF-MS approach was employed to assess the effects of a diet composed of M. micrantha leaves on Coptotermes formosanus workers, with the growth performance of these workers also being assessed. The workers increased their dietary intake when fed M. micrantha leaves, with a concomitant gradual increase in mortality rate. A total of 62 differentially abundant metabolites and nine significantly affected pathways were found when comparing termites fed M. micrantha leaves to pinewood. Key metabolites, including carbohydrates, polyols, 4-hydroxyphenylacetic acid, and their related metabolic pathways, suggested that termites can digest and utilize M. micrantha-derived lignocellulose. However, changes in the tryptophan metabolism, tyrosine metabolism, and sphingolipid metabolism suggest an adverse effect of M. micrantha leaves on antioxidant activity and signal transduction in termites. Overall, this study identified the key metabolites and pathways associated with the response of these termites to dietary changes and the effect of M. micrantha on termites.

Sublethal and lethal effects of the imidacloprid on the metabolic characteristics based on high-throughput non-targeted metabolomics in Aphis gossypii Glover

Sublethal effect considered as an emerging factor to assess the environmental risk of insecticides, which can impact the insects on both physiology and behavior. Lethal exposure can be causing near immediate mortality. Pests are inevitably exposed to sublethal and lethal dose in the agroecosystem following application of pesticides. Insecticides, widely used for the control of insect pests, are irreplaceable in insect pest management. The effects of imidacloprid by the method of high-throughput non-targeted metabolomics was investigated in Aphis gossypii Glover exposed to LC₁₀ and LC₉₀ doses of the imidacloprid, and the control group was treated with the same condition without imidacloprid. Pairwise comparisons showed that 111 metabolites changed significantly, 60 in the LC₁₀ group, and 66 in the LC₉₀ group compared to the control group, while only 16 changes in the LC₁₀ were same with that in LC₉₀ group. Among the changed metabolites, a total of 16 metabolites were identified as potential biomarkers, which represented the most influential pathways including glycolysis and gluconeogenesis, alanine, aspartate, and glutamate metabolism, ascorbate and aldarate metabolism, glutathione metabolism, phenylalanine metabolism, tyrosine metabolism, caffeine metabolism and parkinson's disease (PD), which could account for the sublethal and lethal effects on A. gossypii. These modified metabolic pathways demonstrated that high energy consumption, excitotoxicity and oxidative stress (OS) were appeared in both LC₁₀ and LC₉₀ groups, while PD was detected only in the LC₉₀ group. The results of non-targeted metabolomics revealed the effects of neonicotinoid pesticide exposure on A. gossypii successfully, and provided a deep insight into the influenced physiology by the stress of neonicotinoid pesticide in the insect.

Feeding, hunger, satiety and serotonin in invertebrates

The serotonergic modulation of feeding behaviour has been intensively studied in several invertebrate groups, including Arthropoda, Annelida, Nematoda and Mollusca. These studies offer comparative information on feeding regulation across divergent phyla and also provide general insights into the neural control of feeding. Specifically, model invertebrates are ideal for parsing feeding behaviour into component parts and examining the underlying mechanisms at the levels of biochemical pathways, single cells and identified neural circuitry. Research has found that serotonin is crucial during certain phases of feeding behaviour, especially movements directly underlying food intake, but inessential during other phases. In addition, while the serotonin system can be manipulated systemically in many animals, invertebrate model organisms also allow manipulations at the level of single cells and molecules, revealing limited and precise serotonergic actions. The latter highlight the importance of local versus global modulatory effects of serotonin, a potentially significant consideration for drug and pesticide design.

The behavioral and neurochemical characterization of a Drosophila dysbindin mutant supports the contribution of serotonin to schizophrenia negative symptoms

Mutations in the dystrobrevin binding protein 1 (DTNBP1) gene that encodes for the dysbindin-1 protein, are associated with a higher risk for schizophrenia. Interestingly, individuals carrying high-risk alleles in this gene have been associated with an increased incidence of negative symptoms for the disease, which include anhedonia, avolition and social withdrawal. Here we evaluated behavioral and neurochemical changes in a hypomorphic Drosophila mutant for the orthologue of human Dysbindin-1, dysb¹. Mutant dysb¹ flies exhibit altered social space parameters, suggesting asocial behavior, accompanied by reduced olfactory performance. Moreover, dysb¹ mutant flies show poor performance in basal and startle-induced locomotor activity. We also report a reduction in serotonin brain levels and changes in the expression of the Drosophila serotonin transporter (dSERT) in dysb¹ flies. Our data show that the serotonin-releasing amphetamine derivative 4-methylthioamphetamine (4-MTA) modulates social spacing and locomotion in control flies, suggesting that serotonergic circuits modulate these behaviors. 4-MTA was unable to modify the behavioral deficiencies in mutant flies, which is consistent with the idea that the efficiency of pharmacological agents acting at dSERT depends on functional serotonergic circuits. Thus, our data show that the dysb¹ mutant exhibits behavioral deficits that mirror some aspects of the endophenotypes associated with the negative symptoms of schizophrenia. We argue that at least part of the behavioral aspects associated with these symptoms could be explained by a serotonergic deficit. The dysb¹ mutant presents an opportunity to study the molecular underpinnings of schizophrenia negative symptoms and reveals new potential targets for treatment of the disease.

Tetrahydrobiopterin Plays a Functionally Significant Role in Lipogenesis in the Oleaginous Fungus Mortierella alpina

Tetrahydrobiopterin (BH₄) is well-known as a cofactor of phenylalanine hydroxylase (PAH) and nitric oxide synthase (NOS), but its exact role in lipogenesis is unclear. In this study, the GTP cyclohydrolase I (GTPCH) gene was overexpressed to investigate the role of BH₄ in lipogenesis in oleaginous fungus Mortierella alpina. Transcriptome data analysis reveal that GTPCH expression was upregulated when nitrogen was exhausted, resulting in lipid accumulation. Significant changes were also found in the fatty acid profile of M. alpina grown on medium that contained a GTPCH inhibitor relative to that of M. alpina grown on medium that lacked the inhibitor. GTPCH overexpression in M. alpina (the MA-GTPCH strain) led to a sevenfold increase in BH₄ levels and enhanced cell fatty acid synthesis and poly-unsaturation. Increased levels of nicotinamide adenine dinucleotide phosphate (NADPH) and upregulated expression of NADPH-producing genes in response to enhanced BH₄ levels were also observed, which indicate a novel aspect of the NADPH regulatory mechanism. Increased BH₄ levels also enhanced phenylalanine hydroxylation and nitric oxide synthesis, and the addition of an NOS or a PAH inhibitor in the MA-GTPCH and control strain cultures decreased fatty acid accumulation, NADPH production, and the transcript levels of NADPH-producing genes. Our research suggests an important role of BH₄ in lipogenesis and that the phenylalanine catabolism and arginine-nitric oxide pathways play an integrating role in translating the effects of BH₄ on lipogenesis by regulating the cellular NADPH pool. Thus, our findings provide novel insights into the mechanisms of efficient lipid biosynthesis regulation in oleaginous microorganisms and lay a foundation for the genetic engineering of these organisms to optimize their dietary fat yield.

Phenylethylamides derived from bacterial secondary metabolites specifically inhibit an insect serotonin receptor

Serotonin (5-hydroxytryptamine: 5-HT) is a biogenic monoamine that mediates immune responses and modulates nerve signal in insects. Se-5HTR, a specific receptor of serotonin, has been identified in the beet armyworm, Spodoptera exigua. It is classified into subtype 7 among known 5HTRs. Se-5HTR was expressed in all developmental stages of S. exigua. It was expressed in all tested tissues of larval stage. Its expression was up-regulated in hemocytes and fat body in response to immune challenge. RNA interference (RNAi) of Se-5HTR exhibited significant immunosuppression by preventing cellular immune responses such as phagocytosis and nodulation. Treatment with an inhibitor (SB-269970) specific to 5HTR subtype 7 resulted in significant immunosuppression. Furthermore, knockout mutant of Se-5HTR by CRISPR-Cas9 led to significant reduction of phagocytotic activity of S. exigua hemocytes. Such immunosuppression was also induced by bacterial secondary metabolites derived from Xenorhabdus and Photorhabdus. To determine specific bacterial metabolites inhibiting Se-5HTR, this study screened 37 bacterial secondary metabolites with respect to cellular immune responses associated with Se-5HTR and selected 10 potent inhibitors. These 10 selected compounds competitively inhibited cellular immune responses against 5-HT and shared phenylethylamide (PEA) chemical skeleton. Subsequently, 46 PEA derivatives were screened and resulting potent chemicals were used to design a compound to be highly inhibitory against Se-5HTR. The designed compound was chemically synthesized. It showed high immunosuppressive activities along with specific and competitive inhibition activity for Se-5HTR. This study reports the first 5HT receptor from S. exigua and provides its specific inhibitor designed from bacterial metabolites and their derivatives.

Mapping Neurotransmitter Identity in the Whole-Mount Drosophila Brain Using Multiplex High-Throughput Fluorescence in Situ Hybridization

Identifying the neurotransmitters used by specific neurons is a critical step in understanding the function of neural circuits. However, methods for the consistent and efficient detection of neurotransmitter markers remain limited. Fluorescence in situ hybridization (FISH) enables direct labeling of type-specific mRNA in neurons. Recent advances in FISH allow this technique to be carried out in intact tissue samples such as whole-mount Drosophila melanogaster brains. Here, we present a FISH platform for high-throughput detection of eight common neurotransmitter phenotypes in Drosophila brains. We greatly increase FISH throughput by processing samples mounted on coverslips and optimizing fluorophore choice for each probe to facilitate multiplexing. As application examples, we demonstrate cases of neurotransmitter coexpression, reveal neurotransmitter phenotypes of specific cell types, and explore the onset of neurotransmitter expression in the developing optic lobe. Beyond neurotransmitter markers, our protocols can in principle be used for large-scale FISH detection of any mRNA in whole-mount fly brains.

Big Lessons from Tiny Flies: Drosophila melanogaster as a Model to Explore Dysfunction of Dopaminergic and Serotonergic Neurotransmitter Systems

The brain of Drosophila melanogaster is comprised of some 100,000 neurons, 127 and 80 of which are dopaminergic and serotonergic, respectively. Their activity regulates behavioral functions equivalent to those in mammals, e.g., motor activity, reward and aversion, memory formation, feeding, sexual appetite, etc. Mammalian dopaminergic and serotonergic neurons are known to be heterogeneous. They differ in their projections and in their gene expression profile. A sophisticated genetic tool box is available, which allows for targeting virtually any gene with amazing precision in Drosophila melanogaster. Similarly, Drosophila genes can be replaced by their human orthologs including disease-associated alleles. Finally, genetic manipulation can be restricted to single fly neurons. This has allowed for addressing the role of individual neurons in circuits, which determine attraction and aversion, sleep and arousal, odor preference, etc. Flies harboring mutated human orthologs provide models which can be interrogated to understand the effect of the mutant protein on cell fate and neuronal connectivity. These models are also useful for proof-of-concept studies to examine the corrective action of therapeutic strategies. Finally, experiments in Drosophila can be readily scaled up to an extent, which allows for drug screening with reasonably high throughput.

Biogenic amine signaling systems in the red imported fire ant, Solenopsis invicta - Possible contributors to worker division of labor

The red imported fire ant, Solenopsis invicta Buren, is a dangerous invasive pest in the United States, China and other countries. Efficient division of labor is one of the main reasons for the success of this social insect. Biogenic amines are important regulators of worker division of labor in this eusocial insect, but the related molecular mechanisms are largely unknown. In this study, we identified 10 candidate biogenic amine synthetic enzyme genes and 17 candidate biogenic amine receptor genes in the genome of S. invicta. Quantitative real-time PCR results indicated that foragers had higher head transcripts levels of all the tested enzyme genes than nurses did. In the abdomen, only the rate-limiting enzyme genes for the biosynthesis of serotonin and dopamine were higher in foragers than in nurses. Among the tested serotonin receptors, only the expression of 5-HT_2A gene showed significant difference between foragers and nurses. In the head, more abundant 5-HT_2A transcripts were detected in foragers than in nurses. Foragers expressed higher Octβ4R than nurses in the head and abdomen. However, much lower mRNA levels of Dop3 receptor gene were detected in both body regions of foragers than nurses. Several other octopamine and tyramine receptor genes were also differentially expressed between foragers and nurses in the head and/or in the abdomen. Our results will improve the understanding of molecular mechanisms underlying biogenic amine modulation of the worker division of labor in S. invicta.

Tryptophan hydroxylase (TRH) loss of function mutations induce growth and behavioral defects in Daphnia magna

Tryptophan hydroxylase (TRH) is the rate limiting enzyme in the serotonin synthesis. CRISPR-Cas9 technology was used to generate seven indel TRH mutants in Daphnia magna. Mono-allelic indel TRH-/+ clones showed normal levels of serotonin, measured by both immunohistochemistry and mass spectrometry (LC-MS/MS), whereas bi-allelic indel TRH-/- clones showed no detectable levels of serotonin. Life history and behavioral responses of TRH-/- clones showed the anti-phenotype of those exposed to selective serotonin reuptake inhibitors (SSRI). Mutants lacking serotonin grew less and hence reproduced latter, produced smaller clutches of smaller offspring and responded to a greater extent to light than wild type individuals. Mono-allelic indel TRH-/+ individuals showed the intermediate phenotype. The SSRI fluoxetine enhanced offspring production in all clones and decreased the response to light only in those clones having serotonin, thus indication that behavioral effects of this drug in D. magna are associated to serotonin. Results obtained with the TRH mutants are in line with reported ones in TRH knockouts of Caenorhabditis elegans, Drosophila and mice, indicating that there is one gene encoding TRH, which is the serotonin limiting enzyme in both the central and the periphery nervous system in Daphnia and that deprivation of serotonin increases anxiety-like behavior.

Live fate-mapping of joint-associated fibroblasts visualizes expansion of cell contributions during zebrafish fin regeneration

The blastema is a mass of progenitor cells responsible for regeneration of amputated salamander limbs and fish fins. Previous studies have indicated that resident cell sources producing the blastema contribute lineage-restricted progeny to regenerating tissue. However, these studies have labeled general cell types rather than granular cell subpopulations, and they do not explain the developmental transitions that must occur for distal structures to arise from cells with proximal identities in the appendage stump. Here, we find that regulatory sequences of tph1b, which encodes an enzyme that synthesizes serotonin, mark a subpopulation of fibroblast-like cells restricted to the joints of uninjured adult zebrafish fins. Amputation stimulates serotonin production in regenerating fin fibroblasts, yet targeted tph1b mutations abrogating this response do not disrupt fin regeneration. In uninjured animals, tph1b-expressing cells contribute fibroblast progeny that remain restricted to joints throughout life. By contrast, upon amputation, tph1b⁺ joint cells give rise to fibroblasts that distribute across the entire lengths of regenerating fin rays. Our experiments visualize and quantify how incorporation into an appendage blastema broadens the progeny contributions of a cellular subpopulation that normally has proximodistal restrictions.

Anatomy and behavioral function of serotonin receptors in Drosophila melanogaster larvae

The biogenic amine serotonin (5-HT) is an important neuroactive molecule in the central nervous system of the majority of animal phyla. 5-HT binds to specific G protein-coupled and ligand-gated ion receptors to regulate particular aspects of animal behavior. In Drosophila, as in many other insects this includes the regulation of locomotion and feeding. Due to its genetic amenability and neuronal simplicity the Drosophila larva has turned into a useful model for studying the anatomical and molecular basis of chemosensory behaviors. This is particularly true for the olfactory system, which is mostly described down to the synaptic level over the first three orders of neuronal information processing. Here we focus on the 5-HT receptor system of the Drosophila larva. In a bipartite approach consisting of anatomical and behavioral experiments we describe the distribution and the implications of individual 5-HT receptors on naïve and acquired chemosensory behaviors. Our data suggest that 5-HT_1A, 5-HT_1B, and 5-HT₇ are dispensable for larval naïve olfactory and gustatory choice behaviors as well as for appetitive and aversive associative olfactory learning and memory. In contrast, we show that 5-HT/5-HT_2A signaling throughout development, but not as an acute neuronal function, affects associative olfactory learning and memory using high salt concentration as a negative unconditioned stimulus. These findings describe for the first time an involvement of 5-HT signaling in learning and memory in Drosophila larvae. In the longer run these results may uncover developmental, 5-HT dependent principles related to reinforcement processing possibly shared with adult Drosophila and other insects.

Comparative metabolomics analysis of Callosobruchus chinensis larvae under hypoxia, hypoxia/hypercapnia and normoxia

BACKGROUND

Insect tolerance to low oxygen (hypoxia) and high carbon dioxide (hypercapnia) is critical for insect control. On the basis of bioassay, metabolism profiles were built to investigate adaptive mechanisms in bean weevil under hypoxia (2% O₂ ), hypoxia/hypercapnia (2% O₂ + 18% CO₂ ) and normoxia (control, 20% O₂ + 80% N₂ ) using gas chromatography/time-of-flight mass spectrometry (GC/TOF-MS).

RESULTS

The growth and development of bean weevils were significantly suppressed by the two hypoxia conditions; hypercapnia enhanced the mortality, but after 24 days of exposure, the surviving insects emerged as adults earlier than those under hypoxia only. Metabolism profiles also showed striking differences in metabolites among the treatment and control groups, both quantitatively and qualitatively. Pairwise comparisons of the three groups showed that 61 metabolites changed significantly, 40 in the hypoxia group and 37 in the hypoxia/hypercapnia group relative to the control group, while only 16 were shared equally by the hypoxia and hypoxia/hypercapnia groups. Increased metabolites were mainly carbohydrates, amino acids and organic acids, while free fatty acids were decreased. Furthermore, the changes were strengthened by the addition of hypercapnia, but excluding free fatty acids.

CONCLUSION

The findings show that bean weevil has high tolerance to hypoxia or even hypoxia/hypercapnia at biologically achievable levels and provide more direct evidence for stored product insect mechanism regulation under hypoxia stress, especially free fatty acid regulation by hypercapnia but not by hypoxia. © 2016 Society of Chemical Industry.

Dm5-HT2B: Pharmacological Characterization of the Fifth Serotonin Receptor Subtype of Drosophila melanogaster

Serotonin (5-hydroxytryptamine, 5-HT) is an important regulator of physiological and behavioral processes in both protostomes (e.g., insects) and deuterostomes (e.g., mammals). In insects, serotonin has been found to modulate the heart rate and to control secretory processes, development, circadian rhythms, aggressive behavior, as well as to contribute to learning and memory. Serotonin exerts its activity by binding to and activating specific membrane receptors. The clear majority of these receptors belong to the superfamily of G-protein-coupled receptors. In Drosophila melanogaster, a total of five genes have been identified coding for 5-HT receptors. From this family of proteins, four have been pharmacologically examined in greater detail, so far. While Dm5-HT_1A, Dm5-HT_1B, and Dm5-HT₇ couple to cAMP signaling cascades, the Dm5-HT_2A receptor leads to Ca²⁺ signaling in an inositol-1,4,5-trisphosphate-dependent manner. Based on sequence similarity to homologous genes in other insects, a fifth D. melanogaster gene was uncovered coding for a Dm5-HT_2B receptor. Knowledge about this receptor’s pharmacological properties is very limited. This is quite surprising because Dm5-HT_2B has been attributed to distinct physiological functions based on genetic interference with its gene expression. Mutations were described reducing the response of the larval heart to 5-HT, and specific knockdown of Dm5-HT_2B mRNA in hemocytes resulted in a higher susceptibility of the flies to bacterial infection. To gain deeper understanding of Dm5-HT_2B’s pharmacology, we evaluated the receptor’s response to a series of established 5-HT receptor agonists and antagonists in a functional cell-based assay. Metoclopramide and mianserin were identified as two potent antagonists that may allow pharmacological interference with Dm5-HT_2B signaling in vitro and in vivo.

Commensal bacteria and essential amino acids control food choice behavior and reproduction

Choosing the right nutrients to consume is essential to health and wellbeing across species. However, the factors that influence these decisions are poorly understood. This is particularly true for dietary proteins, which are important determinants of lifespan and reproduction. We show that in Drosophila melanogaster, essential amino acids (eAAs) and the concerted action of the commensal bacteria Acetobacter pomorum and Lactobacilli are critical modulators of food choice. Using a chemically defined diet, we show that the absence of any single eAA from the diet is sufficient to elicit specific appetites for amino acid (AA)-rich food. Furthermore, commensal bacteria buffer the animal from the lack of dietary eAAs: both increased yeast appetite and decreased reproduction induced by eAA deprivation are rescued by the presence of commensals. Surprisingly, these effects do not seem to be due to changes in AA titers, suggesting that gut bacteria act through a different mechanism to change behavior and reproduction. Thus, eAAs and commensal bacteria are potent modulators of feeding decisions and reproductive output. This demonstrates how the interaction of specific nutrients with the microbiome can shape behavioral decisions and life history traits.

Identification and Expression Analysis of Tryptophan Hydroxylase in the Brain and Ventral Nerve Cord of Ragworm Neanthes japonica (Polychaeta, Annelida)

Tryptophan hydroxylase (TPH) was stained in the central nervous system of the Neanthes japonica (Polychaeta, Annelida), using sheep anti-tryptophan hydroxylase antibody by the Streptavidin-Peroxidase immunohistochemical method and Colophony-Paraffin embedded section technique. The immunohistochemistry results revealed that the TPH is distributed in the brain and ventral nerve cord, which is consistent with that of serotonin (5-hydroxytryptamine, 5-HT) that labeled by anti-serotonin antibody. Using the rapid amplification of cDNA ends (RACE) technique, TPH cDNA cloned from Neanthes japonica's central nervous system was 1778bp, which encodes predicted protein of 463 amino acid residues. The co-localization of TPH and 5-HT indicated that the specific TPH was responsible for the central serotonin synthesis in the central nervous system of annelida, TPH and 5-HT not only could be as the novel mutual corroboration marker to detect serotonergic neurons, but also provides the evidences for the evolution of aromatic amino acid hydroxylase genes. Anat Rec, 300:415-424, 2017. © 2016 Wiley Periodicals, Inc.

Serotonin signaling mediates protein valuation and aging

Research into how protein restriction improves organismal health and lengthens lifespan has largely focused on cell-autonomous processes. In certain instances, however, nutrient effects on lifespan are independent of consumption, leading us to test the hypothesis that central, cell non-autonomous processes are important protein restriction regulators. We characterized a transient feeding preference for dietary protein after modest starvation in the fruit fly, Drosophila melanogaster, and identified tryptophan hydroxylase (Trh), serotonin receptor 2a (5HT2a), and the solute carrier 7-family amino acid transporter, JhI-21, as required for this preference through their role in establishing protein value. Disruption of any one of these genes increased lifespan up to 90% independent of food intake suggesting the perceived value of dietary protein is a critical determinant of its effect on lifespan. Evolutionarily conserved neuromodulatory systems that define neural states of nutrient demand and reward are therefore sufficient to control aging and physiology independent of food consumption.

DOI:http://dx.doi.org/10.7554/eLife.16843.001

Limiting the amount of protein eaten, while still eating enough to avoid starving, has an unexpected effect: it can slow down aging and extend the lifespan in many animals from flies to mice. Previous work suggests that how an animal perceives food can also influence how fast the animal ages. For example, both flies and worms actually have shorter lifespans if their food intake is reduced when they can still “smell” food in their environment. However, the sensory cues that trigger changes in lifespan and the molecular mechanisms behind these effects are largely unknown.

Ro et al. therefore asked whether fruit flies recognize protein in their food, and if so, whether such a recognition system would influence how the flies age. Flies that had been deprived of food for a brief period tended to eat more protein than other flies that had not been starved. Ro et al. then revealed that serotonin, a brain chemical that can alter the activity of nerve cells, plays a key role in how fruit flies decide to feed specifically on foods that contain protein. Further experiments revealed also that flies age faster when they are allowed to interact with protein in their diet independently from other nutrients, despite eating the same amount. Disrupting any of several components involved in serotonin signaling protected the flies from this effect and led to them living almost twice as long under these conditions.

Ro et al. propose that the components of the recognition system work together to determine the reward associated with consuming protein by enhancing how much an animal values the protein in its food. As such, it is this protein reward or value – rather than just eating protein itself – that influences how quickly the fly ages. Further work is now needed to understand how the brain mechanisms that allow animals to perceive and evaluate food act to control lifespan and aging.

DOI:http://dx.doi.org/10.7554/eLife.16843.002

Serotonin modulates insect hemocyte phagocytosis via two different serotonin receptors

Serotonin (5-HT) modulates both neural and immune responses in vertebrates, but its role in insect immunity remains uncertain. We report that hemocytes in the caterpillar, Pieris rapae are able to synthesize 5-HT following activation by lipopolysaccharide. The inhibition of a serotonin-generating enzyme with either pharmacological blockade or RNAi knock-down impaired hemocyte phagocytosis. Biochemical and functional experiments showed that naive hemocytes primarily express 5-HT1B and 5-HT2B receptors. The blockade of 5-HT1B significantly reduced phagocytic ability; however, the blockade of 5-HT2B increased hemocyte phagocytosis. The 5-HT1B-null Drosophila melanogaster mutants showed higher mortality than controls when infected with bacteria, due to their decreased phagocytotic ability. Flies expressing 5-HT1B or 5-HT2B RNAi in hemocytes also showed similar sensitivity to infection. Combined, these data demonstrate that 5-HT mediates hemocyte phagocytosis through 5-HT1B and 5-HT2B receptors and serotonergic signaling performs critical modulatory functions in immune systems of animals separated by 500 million years of evolution.

Modulatory Action by the Serotonergic System: Behavior and Neurophysiology in Drosophila melanogaster

Serotonin modulates various physiological processes and behaviors. This study investigates the role of 5-HT in locomotion and feeding behaviors as well as in modulation of sensory-motor circuits. The 5-HT biosynthesis was dysregulated by feeding Drosophila larvae 5-HT, a 5-HT precursor, or an inhibitor of tryptophan hydroxylase during early stages of development. The effects of feeding fluoxetine, a selective serotonin reuptake inhibitor, during early second instars were also examined. 5-HT receptor subtypes were manipulated using RNA interference mediated knockdown and 5-HT receptor insertional mutations. Moreover, synaptic transmission at 5-HT neurons was blocked or enhanced in both larvae and adult flies. The results demonstrate that disruption of components within the 5-HT system significantly impairs locomotion and feeding behaviors in larvae. Acute activation of 5-HT neurons disrupts normal locomotion activity in adult flies. To determine which 5-HT receptor subtype modulates the evoked sensory-motor activity, pharmacological agents were used. In addition, the activity of 5-HT neurons was enhanced by expressing and activating TrpA1 channels or channelrhodopsin-2 while recording the evoked excitatory postsynaptic potentials (EPSPs) in muscle fibers. 5-HT2 receptor activation mediates a modulatory role in a sensory-motor circuit, and the activation of 5-HT neurons can suppress the neural circuit activity, while fluoxetine can significantly decrease the sensory-motor activity.

Identified Serotonin-Releasing Neurons Induce Behavioral Quiescence and Suppress Mating in Drosophila

Animals show different levels of activity that are reflected in sensory responsiveness and endogenously generated behaviors. Biogenic amines have been determined to be causal factors for these states of arousal. It is well established that, in Drosophila, dopamine and octopamine promote increased arousal. However, little is known about factors that regulate arousal negatively and induce states of quiescence. Moreover, it remains unclear whether global, diffuse modulatory systems comprehensively affecting brain activity determine general states of arousal. Alternatively, individual aminergic neurons might selectively modulate the animals' activity in a distinct behavioral context. Here, we show that artificially activating large populations of serotonin-releasing neurons induces behavioral quiescence and inhibits feeding and mating. We systematically narrowed down a role of serotonin in inhibiting endogenously generated locomotor activity to neurons located in the posterior medial protocerebrum. We identified neurons of this cell cluster that suppress mating, but not feeding behavior. These results suggest that serotonin does not uniformly act as global, negative modulator of general arousal. Rather, distinct serotoninergic neurons can act as inhibitory modulators of specific behaviors.

SIGNIFICANCE STATEMENT

An animal's responsiveness to external stimuli and its various types of endogenously generated, motivated behavior are highly dynamic and change between states of high activity and states of low activity. It remains unclear whether these states are mediated by unitary modulatory systems globally affecting brain activity, or whether distinct neurons modulate specific neuronal circuits underlying particular types of behavior. Using the model organism Drosophila melanogaster, we find that activating large proportions of serotonin-releasing neurons induces behavioral quiescence. Moreover, distinct serotonin-releasing neurons that we genetically isolated and identified negatively affect aspects of mating behavior, but not food uptake. This demonstrates that individual serotoninergic neurons can modulate distinct types of behavior selectively.

New insights into the acute actions from a high dosage of fluoxetine on neuronal and cardiac function: Drosophila, crayfish and rodent models

The commonly used mood altering drug fluoxetine (Prozac) in humans has a low occurrence in reports of harmful effects from overdose; however, individuals with altered metabolism of the drug and accidental overdose have led to critical conditions and even death. We addressed direct actions of high concentrations on synaptic transmission at neuromuscular junctions (NMJs), neural properties, and cardiac function unrelated to fluoxetine's action as a selective 5-HT reuptake inhibitor. There appears to be action in blocking action potentials in crayfish axons, enhanced occurrences of spontaneous synaptic vesicle fusion events in the presynaptic terminals at NMJs of both Drosophila and crayfish. In rodent neurons, cytoplasmic Ca(2+) rises by fluoxetine and is thapsigargin dependent. The Drosophila larval heart showed a dose dependent effect in cardiac arrest. Acute paralytic behavior in crayfish occurred at a systemic concentration of 2mM. A high percentage of death as well as slowed development occurred in Drosophila larvae consuming food containing 100μM fluoxetine. The release of Ca(2+) from the endoplasmic reticulum in neurons and the cardiac tissue as well as blockage of voltage-gated Na(+) channels in neurons could explain the effects on the whole animal as well as the isolated tissues. The use of various animal models in demonstrating the potential mechanisms for the toxic effects with high doses of fluoxetine maybe beneficial for acute treatments in humans. Future studies in determining how fluoxetine is internalized in cells and if there are subtle effects of these mentioned mechanisms presented with chronic therapeutic doses are of general interest.

Epistatic and gene wide effects in YWHA and aromatic amino hydroxylase genes across ADHD and other common neuropsychiatric disorders: Association with YWHAE

Monoamines critically modulate neurophysiological functions affected in several neuropsychiatric disorders. We therefore examined genes encoding key enzymes of catecholamine and serotonin biosynthesis (tyrosine and tryptophan hydroxylases-TH and TPH1/2) as well as their regulatory 14-3-3 proteins (encoded by YWHA-genes). Previous studies have focused mainly on the individual genes, but no analysis spanning this regulatory network has been reported. We explored interactions between these genes in Norwegian patients with adult attention deficit hyperactivity disorder (aADHD), followed by gene-complex association tests in four major neuropsychiatric conditions; childhood ADHD (cADHD), bipolar disorder, schizophrenia, and major depressive disorder. For interaction analyses, we evaluated 55 SNPs across these genes in a sample of 583 aADHD patients and 637 controls. For the gene-complex tests, we utilized the data from large-scale studies of The Psychiatric Genomics Consortium (PGC). The four major neuropsychiatric disorders were examined for association with each of the genes individually as well as in three complexes as follows: (1) TPH1 and YWHA-genes; (2) TH, TPH2 and YWHA-genes; and (3) all genes together. The results show suggestive epistasis between YWHAE and two other 14-3-3-genes - YWHAZ, YWHAQ - in aADHD (nominal P-value of 0.0005 and 0.0008, respectively). In PGC data, association between YWHAE and schizophrenia was noted (P = 1.00E-05), whereas the combination of TPH1 and YWHA-genes revealed signs of association in cADHD, schizophrenia, and bipolar disorder. In conclusion, polymorphisms in the YWHA-genes and their targets may exert a cumulative effect in ADHD and related neuropsychiatric conditions, warranting the need for further investigation of these gene-complexes. © 2015 The Authors. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics Published by Wiley Periodicals, Inc.

Serotonin and the search for the anatomical substrate of aggression

All species of animals display aggression in order to obtain resources such as territories, mates, or food. Appropriate displays of aggression rely on the correct identification of a potential competitor, an evaluation of the environmental signals, and the physiological state of the animal. With a hard-wired circuitry involving fixed numbers of neurons, neuromodulators like serotonin offer adaptive flexibility in behavioral responses without changing the "hard-wiring". In a recent report, we combined intersectional genetics, quantitative behavioral assays and morphological analyses to identify single serotonergic neurons that modulate the escalation of aggression. We found anatomical target areas within the brain where these neurons appear to form synaptic contacts with 5HT1A receptor-expressing neurons, and then confirmed the likelihood of those connections on a functional level. In this Extra View article, we offer an extended discussion of these recent findings and elaborate on how they can link a cellular and functional mapping of an aggression-regulating circuit at a single-cell resolution level.

Serotonergic neurons of the Drosophila air-puff-stimulated flight circuit

Monoaminergic modulation of insect flight is well documented. Recently, we demonstrated that synaptic activity is required in serotonergic neurons for Drosophila flight. This requirement is during early pupal development, when the flight circuit is formed, as well as in adults. Using a Ca2+-activity-based GFP reporter, here we show that serotonergic neurons in both prothoracic and mesothoracic segments are activated upon air-puff-stimulated flight. Moreover ectopic activation of the entire serotonergic system by TrpA1, a heat activated cation channel, induces flight, even in the absence of an air-puff stimulus.

Drosophila insulin-producing cells are differentially modulated by serotonin and octopamine receptors and affect social behavior

A set of 14 insulin-producing cells (IPCs) in the Drosophila brain produces three insulin-like peptides (DILP2, 3 and 5). Activity in IPCs and release of DILPs is nutrient dependent and controlled by multiple factors such as fat body-derived proteins, neurotransmitters, and neuropeptides. Two monoamine receptors, the octopamine receptor OAMB and the serotonin receptor 5-HT1A, are expressed by the IPCs. These receptors may act antagonistically on adenylate cyclase. Here we investigate the action of the two receptors on activity in and output from the IPCs. Knockdown of OAMB by targeted RNAi led to elevated Dilp3 transcript levels in the brain, whereas 5-HT1A knockdown resulted in increases of Dilp2 and 5. OAMB-RNAi in IPCs leads to extended survival of starved flies and increased food intake, whereas 5-HT1A-RNAi produces the opposite phenotypes. However, knockdown of either OAMB or 5-HT1A in IPCs both lead to increased resistance to oxidative stress. In assays of carbohydrate levels we found that 5-HT1A knockdown in IPCs resulted in elevated hemolymph glucose, body glycogen and body trehalose levels, while no effects were seen after OAMB knockdown. We also found that manipulations of the two receptors in IPCs affected male aggressive behavior in different ways and 5-HT1A-RNAi reduced courtship latency. Our observations suggest that activation of 5-HT1A and OAMB signaling in IPCs generates differential effects on Dilp transcription, fly physiology, metabolism and social interactions. However the findings do not support an antagonistic action of the two monoamines and their receptors in this particular system.

Pharmacological and genetic identification of serotonin receptor subtypes on Drosophila larval heart and aorta

Serotonin, 5-hydroxytryptamine (5-HT), plays various roles in the fruit fly, Drosophila melanogaster. Previous studies have shown that 5-HT modulates the heart rate in third instar larvae. However, the receptor subtypes that mediate 5-HT action in larval cardiac tissue had yet to be determined. In this study, various 5-HT agonists and antagonists were employed to determine which 5-HT receptor subtypes are responsible for the positive chronotropic effect by 5-HT. The pharmacological results demonstrate that a 5-HT2B agonist significantly increases the heart rate; however, 5-HT1A, 5-HT1B, and 5-HT7 agonists do not have a significant effect on the heart rate. Furthermore, 5-HT2 antagonist, ketanserin, markedly reduces the positive chronotropic effect of 5-HT in a dose-response manner. Furthermore, we employed genetic approaches to confirm the pharmacological results. For this purpose, we used RNA interference line to knock down 5-HT2ADro and also used 5-HT2ADro and 5-HT2BDro insertional mutation lines. The results show that 5-HT2ADro or 5-HT2BDro receptor mutations reduce the response of the heart to 5-HT. Given these results, we conclude that these 5-HT2 receptor subtypes are involved in the action of 5-HT on the heart rate in the larval stage.

Role of the phenylalanine-hydroxylating system in aromatic substance degradation and lipid metabolism in the oleaginous fungus Mortierella alpina

Mortierella alpina is a filamentous fungus commonly found in soil that is able to produce lipids in the form of triacylglycerols that account for up to 50% of its dry weight. Analysis of the M. alpina genome suggests that there is a phenylalanine-hydroxylating system for the catabolism of phenylalanine, which has never been found in fungi before. We characterized the phenylalanine-hydroxylating system in M. alpina to explore its role in phenylalanine metabolism and its relationship to lipid biosynthesis. Significant changes were found in the profile of fatty acids in M. alpina grown on medium containing an inhibitor of the phenylalanine-hydroxylating system compared to M. alpina grown on medium without inhibitor. Genes encoding enzymes involved in the phenylalanine-hydroxylating system (phenylalanine hydroxylase [PAH], pterin-4α-carbinolamine dehydratase, and dihydropteridine reductase) were expressed heterologously in Escherichia coli, and the resulting proteins were purified to homogeneity. Their enzymatic activity was investigated by high-performance liquid chromatography (HPLC) or visible (Vis)-UV spectroscopy. Two functional PAH enzymes were observed, encoded by distinct gene copies. A novel role for tetrahydrobiopterin in fungi as a cofactor for PAH, which is similar to its function in higher life forms, is suggested. This study establishes a novel scheme for the fungal degradation of an aromatic substance (phenylalanine) and suggests that the phenylalanine-hydroxylating system is functionally significant in lipid metabolism.

Neuropeptide-gated perception of appetitive olfactory inputs in Drosophila larvae

Understanding how smell or taste translates into behavior remains challenging. We have developed a behavioral paradigm in Drosophila larvae to investigate reception and processing of appetitive olfactory inputs in higher-order olfactory centers. We found that the brief presentation of appetitive odors caused fed larvae to display impulsive feeding of sugar-rich food. Deficiencies in the signaling of neuropeptide F (NPF), the fly counterpart of neuropeptide Y (NPY), blocked appetitive odor-induced feeding by disrupting dopamine (DA)-mediated higher-order olfactory processing. We have identified a small number of appetitive odor-responsive dopaminergic neurons (DL2) whose activation mimics the behavioral effect of appetitive odor stimulation. Both NPF and DL2 neurons project to the secondary olfactory processing center; NPF and its receptor NPFR1 mediate a gating mechanism for reception of olfactory inputs in DL2 neurons. Our findings suggest that eating for reward value is an ancient behavior and that fly larvae are useful for studying neurobiology and the evolution of olfactory reward-driven behavior.

Serotonin circuits and anxiety: what can invertebrates teach us?

Fear, a reaction to a threatening situation, is a broadly adaptive feature crucial to the survival and reproductive fitness of individual organisms. By contrast, anxiety is an inappropriate behavioral response often to a perceived, not real, threat. Functional imaging, biochemical analysis, and lesion studies with humans have identified the HPA axis and the amygdala as key neuroanatomical regions driving both fear and anxiety. Abnormalities in these biological systems lead to misregulated fear and anxiety behaviors such as panic attacks and post-traumatic stress disorders. These behaviors are often treated by increasing serotonin levels at synapses, suggesting a role for serotonin signaling in ameliorating both fear and anxiety. Interestingly, serotonin signaling is highly conserved between mammals and invertebrates. We propose that genetically tractable invertebrate models organisms, such as Drosophila melanogaster and Caenorhabditis elegans, are ideally suited to unravel the complexity of the serotonin signaling pathways. These model systems possess well-defined neuroanatomies and robust serotonin-mediated behavior and should reveal insights into how serotonin can modulate human cognitive functions.

dFatp regulates nutrient distribution and long-term physiology in Drosophila

Nutrient allocation and usage plays an important part in regulating the onset and progression of age-related functional declines. Here, we describe a heterozygous mutation in Drosophila (dFatp) that alters nutrient distribution and multiple aspects of physiology. dFatp mutants have increased lifespan and stress resistance, altered feeding behavior and fat storage, and increased mobility. Concurrently, mutants experience impairment of cardiac function. We show that endurance exercise reverses increased lipid storage in the myocardium and the deleterious cardiac function conferred by dFatp mutation. These findings establish a novel conserved genetic target for regulating lifespan and physiology in aging animals. These findings also highlight the importance of varying exercise conditions in assessing aging functions of model organisms.

The serotonergic central nervous system of the Drosophila larva: anatomy and behavioral function

The Drosophila larva has turned into a particularly simple model system for studying the neuronal basis of innate behaviors and higher brain functions. Neuronal networks involved in olfaction, gustation, vision and learning and memory have been described during the last decade, often up to the single-cell level. Thus, most of these sensory networks are substantially defined, from the sensory level up to third-order neurons. This is especially true for the olfactory system of the larva. Given the wealth of genetic tools in Drosophila it is now possible to address the question how modulatory systems interfere with sensory systems and affect learning and memory. Here we focus on the serotonergic system that was shown to be involved in mammalian and insect sensory perception as well as learning and memory. Larval studies suggested that the serotonergic system is involved in the modulation of olfaction, feeding, vision and heart rate regulation. In a dual anatomical and behavioral approach we describe the basic anatomy of the larval serotonergic system, down to the single-cell level. In parallel, by expressing apoptosis-inducing genes during embryonic and larval development, we ablate most of the serotonergic neurons within the larval central nervous system. When testing these animals for naïve odor, sugar, salt and light perception, no profound phenotype was detectable; even appetitive and aversive learning was normal. Our results provide the first comprehensive description of the neuronal network of the larval serotonergic system. Moreover, they suggest that serotonin per se is not necessary for any of the behaviors tested. However, our data do not exclude that this system may modulate or fine-tune a wide set of behaviors, similar to its reported function in other insect species or in mammals. Based on our observations and the availability of a wide variety of genetic tools, this issue can now be addressed.

The sea lamprey tryptophan hydroxylase: new insight into the evolution of the serotonergic system of vertebrates

Recent research has shown that at least two tryptophan hydroxylase (Tph) genes are present in gnathostome vertebrates, but it is not known when the duplication of the ancestral Tph gene took place during evolution. By their position as an out-group of gnathostomes, lampreys (agnathans) are key models to understand molecular evolution in vertebrates. Here, we report the cloning of a Tph cDNA of the sea lamprey and the pattern of Tph mRNA expression in larval and postmetamorphic (young adult) sea lampreys using in situ hybridization. Phylogenetic analysis indicated that the lamprey Tph is an orthologue of Tphs of other vertebrates and suggested that the duplication of the ancestral Tph gene occurred before the separation of agnathans and gnathostomes, although alternative hypothesis are also discussed in the present study. In the sea lamprey brain, the Tph transcript was expressed in perikarya of the pineal organ, the retina, the diencephalic and rhombencephalic nuclei reported previously with serotonin immunohistochemistry and in small cells of the spinal cord, with a pattern similar to that observed with anti-serotonin antibodies. This suggests that expression of this Tph gene is shared by all lamprey serotonergic brain populations, unlike that reported in zebrafish and mammals for their different Tph genes. However, no Tph expression was observed in peripheral serotonergic cells, which, unlike in other vertebrates, are widely distributed in lampreys. Our results suggest that the selection of Tph2 to be expressed in raphe neurons may have occurred along the line leading to gnathostomes.

Neurotrophic actions of dopamine on the development of a serotonergic feeding circuit in Drosophila melanogaster

BACKGROUND

In the fruit fly, Drosophila melanogaster, serotonin functions both as a neurotransmitter to regulate larval feeding, and in the development of the stomatogastric feeding circuit. There is an inverse relationship between neuronal serotonin levels during late embryogenesis and the complexity of the serotonergic fibers projecting from the larval brain to the foregut, which correlate with perturbations in feeding, the functional output of the circuit. Dopamine does not modulate larval feeding, and dopaminergic fibers do not innervate the larval foregut. Since dopamine can function in central nervous system development, separate from its role as a neurotransmitter, the role of neuronal dopamine was assessed on the development, and mature function, of the 5-HT larval feeding circuit.

RESULTS

Both decreased and increased neuronal dopamine levels in late embryogenesis during development of this circuit result in depressed levels of larval feeding. Perturbations in neuronal dopamine during this developmental period also result in greater branch complexity of the serotonergic fibers innervating the gut, as well as increased size and number of the serotonin-containing vesicles along the neurite length. This neurotrophic action for dopamine is modulated by the D2 dopamine receptor expressed during late embryogenesis in central 5-HT neurons. Animals carrying transgenic RNAi constructs to knock down both dopamine and serotonin synthesis in the central nervous system display normal feeding and fiber architecture. However, disparate levels of neuronal dopamine and serotonin during development of the circuit result in abnormal gut fiber architecture and feeding behavior.

CONCLUSIONS

These results suggest that dopamine can exert a direct trophic influence on the development of a specific neural circuit, and that dopamine and serotonin may interact with each other to generate the neural architecture necessary for normal function of the circuit.

Serotonin depresses feeding behaviour in ants

Feeding behaviour is a complex functional system that relies on external signals and the physiological state of the animal. This is also the case in ants as they vary their feeding behaviour according to food characteristics, environmental conditions and - as they are social insects - to the colony's requirements. The biogenic amine serotonin (5-HT) was shown to be involved in the control and modulation of many actions and processes related to feeding in both vertebrates and invertebrates. In this study, we investigated whether 5-HT affects nectar feeding in ants by analysing its effect on the sucking-pump activity. Furthermore, we studied 5-HT association with tissues and neuronal ganglia involved in feeding regulation. Our results show that 5-HT promotes a dose-dependent depression of sucrose feeding in Camponotus mus ants. Orally administered 5-HT diminished the intake rate by mainly decreasing the volume of solution taken per pump contraction, without modifying the sucrose acceptance threshold. Immunohistochemical studies all along the alimentary canal revealed 5-HT-like immunoreactive processes on the foregut (oesophagus, crop and proventriculus), while the midgut and hindgut lacked 5-HT innervation. Although the frontal and suboesophageal ganglia contained 5-HT immunoreactive cell bodies, serotonergic innervation in the sucking-pump muscles was absent. The results are discussed in the frame of a role of 5-HT in feeding control in ants.

Identification and expression analysis of the genes involved in serotonin biosynthesis and transduction in the field cricket Gryllus bimaculatus

Serotonin (5-HT) modulates various aspects of behaviours such as aggressive behaviour and circadian behaviour in the cricket. To elucidate the molecular basis of the cricket 5-HT system, we identified 5-HT-related genes in the field cricket Gryllus bimaculatus DeGeer. Complementary DNA of tryptophan hydroxylase and phenylalanine-tryptophan hydroxylase, which convert tryptophan into 5-hydroxy-L-tryptophan (5-HTP), and that of aromatic L-amino acid decarboxylase, which converts 5-HTP into 5-HT, were isolated from a cricket brain cDNA library. In addition, four 5-HT receptor genes (5-HT(1A) , 5-HT(1B) , 5-HT(2α) , and 5-HT(7) ) were identified. Expression analysis of the tryptophan hydroxylase gene TRH and phenylalanine-tryptophan hydroxylase gene TPH, which are selectively involved in neuronal and peripheral 5-HT synthesis in Drosophila, suggested that two 5-HT synthesis pathways co-exist in the cricket neuronal tissues. The four 5-HT receptor genes were expressed in various tissues at differential expression levels, suggesting that the 5-HT system is widely distributed in the cricket.